Forages: Other Crop Problems
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Severe infestations of disease and insects that result in reduced stand vigour, reduced root reserves and slow regrowth will increase the risk of winterkill. Potato leafhopper control can be important in reducing winterkill, particularly in the seeding year (see Potato Leafhopper).
Figure 3-3, Forages Scouting Calendar, shows insects and diseases that could be causing the symptoms in the field. Individual descriptions of insects and diseases, scouting and management strategies can be found in Chapter 13, Insects and Pests of Field Crops, and Chapter 14, Diseases of Field Crops.
Recommended treatments to control insects, pests and diseases can
be found in OMAFRA
Publication 812, Field Crop Protection Guide.
Winterkill of forage stands can cause serious problems on livestock farms and can be a limiting factor in alfalfa production. It can result in lower-quality feed, shortages of feed, disruption of the rotation and additional costs for reseeding lost stands. With forage production, it can sometimes be difficult to optimize the competing demands of quality, yield and persistence. Determine how much forage persistence is needed and manage the risks accordingly.
Causes of winterkill include:
Some forage species are hardier than others. Much of the concern over winterkill centres around alfalfa. The legumes birdsfoot trefoil, red clover, wild white clover and alsike will tolerate more adverse winter conditions than alfalfa or ladino clover. The grasses timothy, reed canarygrass, bluegrass and bromegrass rarely winterkill; thus their use in mixtures gives stand insurance. Orchardgrass and perennial ryegrass are more likely to be killed by icing or low temperatures.
Hardening is the process of cold tolerance development initiated by shorter fall days and cooler temperatures. During the hardening process, plants store carbohydrates in crowns and taproots. The starch is converted to sugars, which gives the plants some protection from freezing. Plants also lose some cellular water to reduce freezing damage. Long fall periods with cool, dry, sunny conditions favour winter hardening.
Critical Fall Harvest Period for Alfalfa
Harvesting before the critical fall harvest period for alfalfa, also known as the "fall rest period," allows the plants to regrow and build sufficient root energy reserves for over-wintering. Adequate root reserves are necessary for winter survival and persistence, as well as for vigorous spring growth and good first-cut yields. The critical fall harvest period is the approximately 6-week window before the historical average killing frost, and varies with location. See Figure 3-4, Start of the Critical Fall Harvest Period for Alfalfa, to determine the critical fall harvest period in your area. The risk of alfalfa winterkill increases by harvesting during this period and should be weighed against the immediate need for forage. Yield sacrificed by not harvesting during the critical fall harvest period is usually easily regained in the first-cut yield the following year. See the OMAFRA Factsheet, Risk of Alfalfa Winterkill, Order No. 91-072. or visit the OMAFRA website at www.ontario.ca/crops.
To ensure persistence and succeeding yields, consider the risks before harvesting alfalfa during the 6-week period following the date shown on the map for each region.
Winter-Tolerant Alfalfa Varieties
See the Ontario Forage Crop Variety Performance brochure available at OMAFRA Resource Centres or at www.uoguelph.ca/plant/performance_recommendations/ofcc/ofcc.htm. High alfalfa yield indexes in the third and fourth year indicate high relative persistence. This is a composite measure of a number of production characteristics, including winter-hardiness, cold tolerance, disease resistance and yield. In general, varieties that have "resistant" or "highly resistant" ratings for disease have a lower risk of winterkill.
Adequate snow cover of at least 15 cm (6 in.) insulates the alfalfa crown and root at moderate temperatures. A lack of snow cover can expose alfalfa crowns to temperatures less than -15°C. This results in freezing damage to plant cells and eventual plant death. The insulation effect from snow also reduces soil temperature fluctuations and risk from heaving. Fluctuating winter temperatures with lows below freezing and highs greater than 5ºC, without snow cover, can cause plants to break dormancy and become more susceptible to freezing.
Fast melting of snow followed by cold temperatures can result in ice sheeting, which smothers the plants by restricting oxygen. Ice sheeting also causes freezing damage to alfalfa crowns, due to the poor insulating ability of ice.
Leaving Fall Growth to Hold Snow
After a hard fall frost, alfalfa can be harvested without lowering root reserves, but there are disadvantages. Leaving the fall growth will help catch snow, which insulates the soil from cold temperatures. The stubble also helps alfalfa plants survive ice sheeting by protruding through the ice, allowing the movement of air for respiration. Unlike grasses, fall regrowth of alfalfa does not cause "smothering," however, ice sheeting does.
Both surface and subsurface drainage are essential to alfalfa winter survival. Heaving can occur on heavy soils with poor drainage since repeated freezing and thawing cause the taproot to be pushed out of the soil (see Plate 22). Plants heaving more than 2.5 cm (1 in.) will dry out, have broken taproots and lateral roots, and elevated crowns. These plants will die or be severely stunted. Surface drainage is a bigger issue on flat land than rolling land, because with frozen ground, water cannot drain off the field, resulting in ice sheeting.
Frequent harvests with short cutting intervals put stress on alfalfa that can reduce winter survival and first-cut yield the following year. Conversely, a long interval between cuttings will rebuild plant reserves and enhance winter survival. Some growers place more emphasis on high-yielding, frequently cut, high-quality stands that last for 3 years and are less concerned about alfalfa persistence. Intervals of less than 35 days between cuts increases the risk of winterkill. Allowing the final cut to show some flower before harvest to ensure adequate root reserves can be a compromise between quality and winter survival.
Soil Fertility and pH
A low soil-potassium level is a major factor leading to loss of alfalfa in a stand, particularly on loams and sandy loams. Adequate soil potassium levels allow alfalfa to store sufficient carbohydrates in the roots to resist low-temperature injury and act as the energy source for vigorous spring growth. Soil-sample forage fields regularly and fertilize them according to recommendations. Apply the fertilizer before the start of the critical fall harvest period to allow for sufficient plant uptake.
However, high soil-potassium levels can result in luxury consumption of potassium by alfalfa and subsequent nutritional problems when fed to dairy cows prior to calving. Potassium applications on soils testing over 150 ppm will not significantly increase winter hardiness and are not recommended.
Boron deficiency can also lead to stunted alfalfa growth during
midsummer dry periods and thus weaker plants going into winter.
For soils low in pH, add lime the year prior to seeding (see Fertility
Assessing an Alfalfa Stand for Winter Survival
Future yield potential can be estimated by counting the number of plants or stems per square metre, but the health of crowns and roots is extremely important. Stem counts are more accurate than plant counts, but in early spring it may only be possible to count the number of crowns. Be prepared to replace an older stand if it has less than 43 plants/m2 (4 plants/ft2). See Table 3-14, Desirable Alfalfa Stand Plant Count.
Dig several plants to determine the health of the crown and root.
Healthy crowns are large and symmetrical and have many shoots. Cut
a root open lengthwise. Healthy roots will have a white or creamy
colour inside, and are firm and resistant to pealing when scratched
with a thumbnail. Dying plants will have a discoloured crown and
root and a spongy texture. Check for bud or new shoot vigour. Plants
with broken taproots from heaving may green up but die later. Slightly
heaved plants can survive, but their longevity and productivity
will be reduced.
Source: Undersander and Cosgrove, University of Wisconsin, 1992
When alfalfa is about 15 cm (6 in.) in height, stems/m2 (stems/ft2) can be used as the density measure. Stem density of 590 stems/m2 (55 stems/ft2) has good yield potential (see Figure 3-5, Alfalfa Yield Potential at Various Stem Count Densities). There may be some yield loss with stem counts between 431-539 plants/m2 (40-50 plants/ft2).
Consider replacing the stand if there are less than 430 stems/m2 (40 stems/ft2) and the crown and root health is poor.
Other considerations for keeping or retiring a forage stand include forage inventories and requirements, alternate forage options, how much grass is left in the stand, rotational requirements and weed pressure.
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